Backscatter inspection portal

ABSTRACT

A system and method for inspecting an object with multiple sources of substantially coplanar penetrating radiation. Irradiation of the inspected object by the sources is temporally sequenced such that the source of detected scattered radiation is unambiguous. Thus, multiple views of the inspected object may be obtained and image quality may be enhanced.

The present application is a Continuation Application of U.S. patentapplication Ser. No. 11/097,092, issued Jul. 15, 2008 as U.S. Pat. No.7,400,701, and, like that Application, claims priority from U.S.Provisional Application No. 60/561,079, filed Apr. 9, 2004. Both of theforegoing applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to systems and methods for inspectingobjects with penetrating radiation, and, more particularly, theinvention relates to inspection systems employing multiple sources ofradiation.

BACKGROUND ART

It is desirable to determine the presence of objects, such ascontraband, weapons, or explosives, that have been concealed, forexample, in a moving vehicle, or on a person, or in any inspectedobject, while the inspected object is moved past one or more systemsthat image the contents of the object using penetrating radiation. Thedetermination should be capable of being made while the inspected objectis in motion, or, alternatively, while the inspection system is inmotion with respect to the inspected person or object. Indeed, sinceinspection rate, and thus hourly throughput, is at a premium, it isdesirable that the vehicle, for example, be driven without requiring thedriver or passengers to alight. In case a detection is made, a visualimage should be available for verification.

The use of images produced by detection and analysis of penetratingradiation scattered from an irradiated object, container, or vehicle isthe subject, for example, of U.S. Pat. No. 6,459,764, to Chalmers et al.(the “Chalmers Patent”), issued Oct. 1, 2002, and incorporated herein byreference. The Chalmers Patent teaches backscatter inspection of amoving vehicle by illuminating the vehicle with x-rays from above orbeneath the moving vehicle, as well as from the side.

The use of an x-ray source and an x-ray detector, both located in aportal, for purposes of screening personnel, is the subject, forexample, of U.S. Pat. No. 6,094,472, to Smith, issued Jul. 25, 2000.

X-rays are scattered from matter in all directions, therefore, scattermay be detected by an x-ray detector disposed at any angle to thescattering material with respect to the direction of incidence of theilluminating radiation. Therefore, a “flying spot” irradiation system istypically used, whereby a single point on the inspected object isilluminated with penetrating radiation at any given moment, so that thelocus of scatter can be determined unambiguously, at least with respectto the plane transverse to the direction of the beam of penetratingradiation.

In order to obtain multiple views of an inspected object, multiplebackscatter imaging systems may be employed in a single inspectiontunnel. This may result in interference, or cross-talk, betweenrespective imaging systems, resulting in image degradation. This is dueto the lack of each flying-spot imager's ability to distinguish theorigin of the scattered radiation from each imager's source. To date,this problem has been addressed by placing the imagers some distanceapart to minimize cross talk. This approach causes the size of theoverall system to increase. In space-limited applications, this is oftenundesirable.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, there is provided aninspection system for inspecting an object. The inspection system has afirst source for providing a first beam of penetrating radiation ofspecified cross-section directed in a first beam direction and a secondsource for providing a second beam of penetrating radiation in a secondbeam direction, and may have additional sources of additional beams. Thebeams of penetrating radiation are temporally interspersed.Additionally, the system has a plurality of scatter detectors fordetecting radiation scattered from at least one of the first beam andthe other beams by any scattering material within the inspected objectand for generating a scattered radiation signal. The system may alsohave one or more transmission detectors for detecting penetratingradiation transmitted through the object. Finally, the system has acontroller for creating an image of the scattering material based atleast on the scattered radiation signal or for otherwise characterizingthe scattering material.

In accordance with alternate embodiments of the invention, the firstsource of penetrating radiation may be an x-ray source, as may the othersources of penetrating radiation. The first beam direction and thedirection of any other beam may be substantially coplanar. The varioussources may include a beam scanning mechanism, such as a rotatingchopper wheel or an electromagnetic scanner, and one or more of thebeams may be pencil beams.

In accordance with yet further embodiments of the invention, emission ofpenetrating radiation in the first beam may be characterized by a firsttemporal period and emission of penetrating radiation in the second beammay be characterized by a second temporal period, the first and thesecond temporal periods offset by fixed phase relationship. The temporalperiod of each source may be characterized by a duty cycle, and theemission of adjacent sources may be characterized by a phaserelationship with respect to an adjacent source, where the phaserelationship may equal to 2π times the duty cycle.

In accordance with yet further embodiments of the invention, theinspection system may further including a display for displaying ascatter image of material disposed within the inspected object.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying drawings, in which:

FIG. 1 shows a schematic cross sectional view of an x-ray inspectionsystem that uses multiple backscatter imaging systems in accordance withembodiments of the present invention; and

FIG. 2 shows a side view of the x-ray inspection system embodiments ofFIG. 1.

FIG. 3 shows a prior art backscatter system employing a rotating chopperwheel of a sort employed in various embodiments of the presentinvention.

FIG. 4 shows a prior art backscatter system employing an electromagneticscanner of a sort employed in various embodiments of the presentinvention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In accordance with embodiments of the present invention, beam cross talkis minimized between or among multiple flying-spot backscatter imagingsystems configured as a multi-view backscatter inspection system, withno restriction on the distance between the individual imaging systems.In other words, in a multi-view system comprised of individualbackscatter imaging systems for each view, the individual imagingsystems can be placed as close together as is physically possible, whilecross talk is advantageously reduced or eliminated.

Methods and advantages of backscatter inspection of a moving vehicle byilluminating the vehicles with x-rays from either above or beneath themoving vehicle are described in U.S. Pat. No. 6,249,567, issued Jun. 19,2001, which is incorporated herein by reference. In an embodiment of theaforesaid patent, illustrated in FIG. 3, rotating chopper wheel 20 isused to develop a pencil beam 14 which may be swept in a planesubstantially parallel to that of the page. The formation of pencil beam14 by a series of tubular collimators 313 distributed as spokes onrotating wheel 20 is known in the art. In accordance with preferredembodiments of the present invention, regions of enhanced backscatterthat arise due to materials concealed close to the side walls of avehicle are revealed without requiring that penetrating radiationtraverse the vehicle during the course of inspection.

FIG. 1 shows a schematic cross-sectional view of the elements of aninspection system, designated generally by numeral 10. An object ofinspection 18, which may be animate or inanimate, moves, or is moved, ina direction into, or out of, the page and thus traverses a portal 12.Portal 12 supports a plurality of sources 13, 15, and 17 of penetratingradiation. Sources 13, 15, and 17 are typically x-ray tubes having beamforming and steering mechanisms known in the art. For example, source 13emits penetrating radiation in a beam 23 having a cross-section of aspecified shape. For scatter imaging applications, a narrow pencil beamis typically employed. Beam 23 of penetrating radiation, may be, forexample, a beam of x-rays such as a polychromatic x-ray beam. Whilesource 13 of penetrating radiation is preferably an x-ray tube, forexample, however other sources of penetrating radiation, such as a linac(linear accelerator), are within the scope of the present invention,and, indeed, the penetrating radiation is not limited to x-ray radiationand may include gamma ray radiation.

A scanning mechanism is provided for scanning beam 23 along asubstantially vertical axis, such that, during a portion of a dutycycle, beam 23 is directed in a series of directions such as 24. Object18 that is to be inspected moves past beam 23 in a substantiallyhorizontal direction, into the page, in the depiction of FIG. 1. Inalternate embodiments of the invention, the source and/or other portionsof the inspection system may be moved in relation to object 18, whichmay be moving itself, or stationary.

Source 13 may include a scanning mechanism such as a flying spotrotating chopper wheel 20 (shown in FIG. 3) as known to persons skilledin the art. Alternatively, electromagnetic scanners may be employed,such as scanner 104 (shown in FIG. 4) those described in U.S. Pat. No.6,421,420, issued Jul. 23, 2002 and entitled “Method and Apparatus forGenerating Sequential Beams of Penetrating Radiation,” which isincorporated herein by reference. A source 412 supplies a beam ofcharged particles 140 that are accelerated to a surface of a target 160.Electromagnetic beam director 418 can be any electromagnetic beamdirecting arrangement such as magnetic or electrostatic yokes.Penetrating electromagnetic radiation is emitted by target 160 and passthrough a collimator 422 disposed a specified distance from the target,thus producing sequential parallel beams of radiation.

Beams of sources 15 and 17 are shown in typical extremal positions oftheir respective scans, and are labeled 25, 26, 27, and 28. Inspectedobject 18, which, as discussed, may refer to a vehicle, a container, ora person, for example, may be self-propelled through beams 23-28 or maybe conveyed by a mechanized conveyor 29 or pulled by a tractor, etc. Inalternate embodiments of the invention, the inspection system,configured, for example, as a portal, may move, or be moved, over anobject such as a vehicle that may, itself, be moving or stationary.

Beams 23-28 will be referred to in the present description, withoutlimitation, as x-ray beams. In accordance with preferred embodiments ofthe invention, a rotating chopper wheel is used to develop a pencil beam23-28 which may be swept in a plane substantially parallel to that ofthe page. The cross section of pencil beam 23 is of comparable extent ineach dimension and is typically substantially circular, although it maybe many shapes. The dimensions of pencil beam 23-28 typically define thescatter image resolution which may be obtained with the system. Othershapes of beam cross section may be advantageously employed inparticular applications.

A detector arrangement, typified by scatter detector 31, is disposed ina plane parallel to the direction of motion of object 18 during thecourse of the scan. X-rays 30 scattered by Compton scattering out ofbeam 24 in an essentially backward direction are detected by one or morebackscatter detectors 31 disposed between source 13 and object 18.Additional detector arrangements 32, 33, 34, 35, and 36 may be usedsupplementarily for detecting x-rays Compton-scattered from beam 24 andsimilarly, as will presently be described, for each of the other beamsincident, in turn, on inspected object 18.

Additionally, transmission detectors disposed distally to the inspectedobject 18 with respect to the emitting source may be used to augment thescatter image or images with an image of the object as obtained intransmitted x-rays, for example, the detector elements designated 35 and36 detect the emission of source 13 as transmitted through the inspectedobject. In another embodiment of the invention, a single separatedetector is disposed between the pair of scatter detectors 35 and thepair of scatter detectors 36 and is employed for detection ofpenetrating radiation transmitted through object 18.

Within the scope of the invention, any x-ray detection technology knownin the art may be employed for the detector arrangements 31-36. Thedetectors may be scintillation materials, either solid or liquid orgaseous, viewed by photo-sensitive detectors such as photomultipliers orsolid state detectors. Liquid scintillators may be doped with tin orother element or elements of high atomic number. Respective outputsignals from the scatter detectors 31-36 are transmitted to a processor40, and processed to obtain images of feature 42 inside the inspectedobject 18. Since incident x-ray photons are scattered by scatteringsources within object 18 into all directions, detectors with large areasare used to maximize the collection of the scattered photons. Inaccordance with certain embodiments of the invention, processor 40(otherwise referred to herein as a ‘controller’) may also be employed toderive other characteristics of the scattering object, such as its mass,mass density, effective atomic number, etc., all as known in the art.

In order to allow views of the inspected object from multipledirections, multiple sources 13-17 are used to irradiate the inspectedobject. However, since the photons emitted by each source are scatteredin all directions, care must be exercised in order to eliminatecross-talk, i.e., the misidentification of the source of irradiation. Inaccordance with embodiments of the present invention, cross talk isadvantageously reduced or eliminated by ensuring that only one imager isemitting radiation at a time. First, the duty-cycle of the beams emittedfrom the imaging systems is set less-than or equal-to the inverse of thenumber of imaging systems, or views, in the multi-view system. Forexample, if the number of views desired is six, each imaging system isset for a duty cycle of ⅙, or less.

Next, the phase relationship between each pair of adjacent sources isset to 2π times the duty cycle. This results in sequenced radiationemission from the imagers, eliminating the possibility of concurrentemission from more than one imager. For example, a multi-view inspectionsystem with 6 sources would require that they run at the same frequency,that their duty-cycles be ⅙, and that their phase relationship be 2π/6,or 60 degrees.

In cases where flying-spot systems are realized by mechanical means suchas rotating hoops and chopper wheels, these aforesaid criteria may bemet by synchronization of the motion of the mechanical chopper elements,biased by phase offsets. Thus, for example, where collimators arerotated to define the path of emergent x-ray beam 23, close-loop motioncontroller systems known in the art may be employed to drive therotation of the collimators. The duty cycle is controlled by setting thefan aperture (the total sweep angle of a beam, i.e., the angle betweenextremal beams 23 and 24 of a single source), equal to 2π times the dutycycle. In systems where the emitted radiation can be controlledelectronically, any desired sequence of irradiation or range of sweepmay be set, without limitation, entirely by electronic or softwarecontrol.

By virtue of temporal sequencing which reduces or eliminates cross-talk,sources may be placed in greater proximity than otherwise possible. Inparticular, sources 13-17 may be disposed in a single plane, whichadvantageously permits virtually simultaneous on/off control of thex-rays regardless of the speed with which the object is passing by theimagers.

The system described may advantageously provide for an image to bederived from the perspective of each successive source 13-17. FIG. 1shows an exemplary three-view system, with beams 23, 25, etc. eachsweeping trajectories that are coplanar.

The beams from each imager sweep in sequence, such that no more than oneimager is emitting radiation at a time. Thus, source (or ‘imager’) 13sweeps its beam first. Radiation scattered from the object, asrepresented by rays 44, is received by all of the detectors. The signalsfrom each of the detectors are acquired as separate channels by anacquisition system. This process is repeated for each of the threeimagers, creating “slices” of the object as it moves by.

Referring now to FIG. 2, a side view is shown of the arrangement of FIG.1, with elements designated by corresponding numbers. A slot 50 is shownthrough which the beam of source 13 passes through segments 52 and 54 ofdetector 31 as object 18 is scanned while moving in a lateral direction16.

The signals from the detectors can be selectively used to reconstruct animage of the object. Since scattered photons 44 detected by detectors 33and 34 from source 13 are as useful as scattered photons from source 17,these same detectors can be shared among all sources, and result inimproved scatter collection with efficient use of the detector hardware.

Embodiments of this invention, furthermore, may advantageously allowmulti-view Flying-Spot X-ray Scatter imaging to be practiced in asmaller operational footprint by eliminating cross talk, and by allowingcloser positioning of the individual imagers for each view. The closepositioning of these imagers (where an “imager” refers to a source, atleast one detector, and associated electronics and signal processing)may also allow sharing of scatter detectors between, or among, imagers,allowing more scatter collection for improved image quality, withefficient use of detector hardware.

In applications where scanning of selective regions of the object isdesired, co-planar positioning of the imagers allows simultaneous on/offcontrol of the x-rays regardless of the speed with which the object ispassing by the imagers. This greatly simplifies the design of thecontrol of x-ray emissions from each imager in the multi-view inspectionsystem, thus individual sequencing of x-ray emissions need not beperformed as is typically practiced in systems in which emission is notco-planar.

Besides imaging contents of concealing enclosures, in terms of whichembodiments of the present invention have been described, othercharacteristics of inspected objects may be obtained within the scope ofthe present invention. For example, backscatter techniques may beapplied, as known in the art, for deriving mass, mass density, massdistribution, mean atomic number, or likelihood of containing targetedthreat material.

In accordance with certain embodiments of the invention, x-rays havingmaximal energies in the range between 160 keV and 300 keV are employed.At this energy, x-rays penetrate into a vehicle, and organic objectsinside the vehicle can be detected. Since lower doses of x-rayirradiation are thus possible, automobiles may be scanned using thepresent invention. For applications where the scanned vehicle maycontain personnel, end point energies below 300 keV are preferred. Thescope of the present invention, however, is not limited by the range ofpenetrating photons employed.

The described embodiments of the invention are intended to be merelyexemplary and numerous variations and modifications will be apparent tothose skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inthe appended claims.

1. An inspection system for inspecting an object, the system comprising:a. a first source for providing a first beam of penetrating radiation ofspecific cross-section directed in a first beam direction; b. a secondsource for providing a second beam of penetrating radiation of specificcross-section directed in a second beam direction substantially coplanarwith the first beam direction and temporally interspersed with the firstbeam of penetrating radiation; c. at least one scatter detector fordetecting radiation scattered from at least one of the first beam andthe second beam by any scattering material within the inspected objectand for generating a scattered radiation signal; d. a controller forcreating a scatter image of the scattering material based at least onthe scattered radiation signal; e. a third source for providing a thirdbeam of penetrating radiation; and f. a transmission detector disposeddistally to the object with respect to the third source for imaging theobject in transmitted x-rays to augment the scatter image.
 2. Theinspection system as set forth in claim 1, wherein the third source isan x-ray source.
 3. The inspection systems as set forth in claim 1,wherein the third source includes a beam scanning mechanism.
 4. Theinspection system as set forth in claim 1, further comprising at leastone transmission detector for detecting at least one of the first beamand the second beam as transmitted through the inspected object and forgenerating a transmitted radiation signal.
 5. The inspection system asset forth in claim 1, wherein the third source is disposed above theinspected object.